This invention relates to novel tertiary-alkylperoxy alkyl carbonate cure initiators that are useful for the vulcanization of silicone rubber, especially hot air vulcanization; to silicone rubber compositions containing the initiators; and to the use of the initiators in vulcanization of silicone rubber, particularly hot air vulcanization.
Silicone rubbers are organopolysiloxane elastomers which perform well at temperature extremes and have exceptional resistance to solvents and compression set. They are used extensively in aerospace, electrical, appliance, and automotive applications. Typical parts made of silicone rubbers include gaskets; O-rings; cushions; sparkplug boots; hoses; seals for transmissions; self-cleaning ovens; and dishwashers; insulation for electrical wire and cable; and molded or extruded parts for electrical components.
Some silicone rubbers, such as bathtub sealant, are self-curing at room temperature, but others must be vulcanized by heating. The latter types are used with organic peroxide curing agents or initiators. Vulcanization may be accomplished in a variety of ways. Molded parts, formed by compression, injection, or transfer molding, are vulcanized in a heated mold. Calendered sheet may be wrapped on a hollow drum and exposed to steam. Duct and hose formed on a mandrel may be vulcanized in an autoclave or steam vulcanizer. Extrusions are generally vulcanized in continuous hot air vulcanization units. Extrusions can also be vulcanized in continuous hot liquid vulcanization units or, in small sections, in autoclaves.
Peroxide initiators used in vulcanization undergo thermal decomposition to form free radicals that cause crosslinking reactions between organic side groups on the silicone polymers. Among the organic peroxides reportedly in commercial use for silicone rubber vulcanization are, for example, 2,4-dichlorobenzoyl peroxide, benzoyl peroxide, dicumyl peroxide, tertiary-butyl perbenzoate, di-tertiary-butyl peroxide, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and tertiary-butylperoxy isopropyl carbonate. Each initiator has particular advantages and limitations depending on the vulcanizing method used, the nature of the vulcanizable composition, and the properties desired in the vulcanizate.
Continuous hot air vulcanization is the most convenient method of vulcanizing extruded silicone rubber articles such as oven seals, tubing, and electrical insulation. Until now, 2,4-dichlorobenzoyl peroxide has been the recommended initiator for general use in this process. Benzoyl peroxide is not generally recommended because it tends to produce porous vulcanizates when the rubber compound is heated without external pressure. Dicumyl peroxide can be used in rubbers containing carbon black filler, but its performance is not as good as that of 2,4-dichlorobenzoyl peroxide. It is used preferably in low concentrations to produce rubbers with the low compression set desirable in gaskets and seals, but at low concentrations it may leave a skin of tacky, incompletely vulcanized rubber composition on extrudate surfaces exposed to air or steam during vulcanization. Other initiators, such as tertiary-butylperoxy isopropyl carbonate, leave an uncured skin so thick that it can easily be scraped off with a fingernail.
Although 2,4-dichlorobenzoyl peroxide is recommended for use with all silicone and fluorosilicone rubbers in the hot air vulcanization process, it has a number of serious disadvantages. It does not give acceptable results in rubbers containing carbon black filler. It is among the highest priced of the available initiators, costing almost two and one half times as much as the next lower priced initiator. Scorch or premature vulcanization can be a problem, especially in thin moldings. Of the peroxides used in curing silicone rubber, it is among the least stable, as measured by its ten hour half-life temperature, which is defined herein to be the temperature at which one-half of the peroxide in the 0.2 molar benzene solution decomposes in ten hours. The lower the half-life temperature, the less stable the peroxide. Therefore cooler milling, and sometimes cooling of the extruder barrels, is required for rubber compounds containing 2,4-dichlorobenzoyl peroxide. This peroxide is also sensitive to shock and friction. Consequently, manufacturing and handling can be hazardous, especially when the peroxide is dried enough to prevent undesirable porosity in the vulcanizate.
U.S. Pat. No. 2,374,789 discloses alkylperoxy alkyl carbonates generally and as polymerization catalysts. Tertiary-butylperoxy cyclohexyl carbonate has been reported in the chemical literature, Journal of Organic Chemistry, Vol. 11, page 3446 (1969). In addition, certain tertiary-alkylperoxy alkyl carbonates are known as initiators for the vulcanization of silicone rubber. U.S. Pat. No. 3,313,762 to Pfeifer discloses a class of these peroxy carbonates for use in the vulcanization of molded silicone rubber articles. The peroxy carbonates are disclosed to have substantial advantages over benzoyl peroxide and dicumyl peroxide in producing vulcanizates with superior toughness, resistance to deterioration from heat aging, and low compression set, which is a permanent deformation of rubber after it has been compressed for a time at high temperature. In addition, the peroxy carbonates are said to allow vulcanization at temperatures significantly lower than those required for dicumyl peroxide. However, the patent does not disclose any use of tertiary-alkylperoxy alkyl carbonates in a hot air vulcanization process. Furthermore, all the working examples in the patent show the use of t-butylperoxy isopropyl carbonate, which, as stated above, is unsatisfactory for use in hot air vulcanization.
Tertiary-butylperoxy isopropyl carbonate is in some respects typical of the peroxy carbonate initiators disclosed in the Pfeifer patent. It can be used with all types of fillers. It is much less sensitive to friction, mechanical, or thermal shock than 2,4-dichlorobenzoyl peroxide. Its ten hour half-life temperature as herein above defined is about 99.degree. C.; the half-life temperature of 2,4-dichlorobenzoyl peroxide is 54.degree. C., indicative of the latter's much lower stability. Other tertiary-alkylperoxy alkyl carbonates, including those of the present invention, have similar properties. Furthermore, the tertiary-alkylperoxy alkyl carbonates of the present invention have higher flash points, and are therefore easier and safer to handle than tertiary-butylperoxy isopropyl carbonate. They may also be dried more fully than 2,4-dichlorobenzoyl peroxide with greater safety. Initiators with the advantages of tertiary-butylperoxy isopropyl carbonate would be desirable for use in the hot air vulcanization process and similar processes.
It has now been discovered that certain novel tertiary-alkylperoxy alkyl carbonates can be employed as initiators in the hot air vulcanization of silicone rubber compounds, as well as in mold and other vulcanization methods, to provide vulcanizates having low porosity, no objectionable odor, and a smooth, firm, completely cured surface. The novel initiators are characterized as having a total of at least 12 carbons in the alkyl portions of the molecule.